Magnetic resonance tomography (MRT) is an imaging method which permits the high-resolution generation of sectional images of living organisms, such as human beings. The patient is positioned in a homogeneous magnetic field B0. Gradient coils are used to modify the external magnetic field in the FOV (field of view) such that firstly a body layer is selected and secondly position encoding of the magnetic resonance (MR) signals generated is effected. In the subsequent reconstruction of the MR signals, for example, by Fourier transformation, an image is produced of the selected layer, which is used for medical diagnostics. The MR signals are generated and detected using a high-frequency system which comprises a transmitting antenna which radiates high-frequency (HF) excitation pulses into the patient, and a receiving antenna which detects the emitted HF resonance signals and forwards them for image reconstruction. By selecting a suitable pulse sequence, such as a spin echo sequence or a gradient echo sequence, and the associated sequence parameters, the contrast of the MR images can be diversely varied depending on the diagnostic task. The MRT maps body structures and accordingly represents a structural imaging method.
Movements during an MR recording, for example respiratory movements of a patient who is to be examined using MR, can in magnetic resonance imaging, especially during an examination of the organs of the thorax and of the abdomen, particularly of examination areas affected by the respiratory movement of the patient, result in artifacts, such as ghosting, blurring and/or a loss of intensity in the images generated for example and in registration errors between images generated. These artifacts may make it more difficult to base a diagnosis on these images, e.g. on the part of a physician, with the result that e.g. lesions may be overlooked.
Numerous techniques exist in the prior art for reducing artifacts caused by, for example, a respiratory movement. One of these techniques is to trigger a trigger signal for capturing magnetic resonance image data as a function of a respiratory movement or generally what is known as respiratory gating. Respiratory gating is a technique in which during the MR measurement, the patient's breathing is captured and is assigned to the measurement data acquired. In respiratory gating, the measurement data is only used for reconstruction if the respiratory movement captured meets certain pre-definable criteria.
The patient's breathing can in this case be detected using external sensors, e.g. a pneumatic cushion, or using MR signals, known as navigators. A navigator is generally a short sequence which acquires MR signals e.g. from the diaphragm or another signal source in the examination object, the movement of which is correlated with the patient's breathing. The respiratory movement can be traced by means of the position of the diaphragm or of the other signal source.
In respiratory gating using navigators, the navigator sequence is interleaved for example with the imaging sequence and a diaphragm position measured using a navigator is then assigned to the imaging data acquired immediately afterward (or beforehand).
A distinction is made between retrospective and prospective respiratory gating. In retrospective respiratory gating, the respiratory movement is captured and plotted during the MR measurement, but not analyzed. Instead the k space to be captured is measured several times. For reconstruction, only part of the measured data is used, preferably that part in which the respiratory signal lies in a particular window around a marked respiratory position. If a particular k space data point required for image reconstruction has been measured several times within the marked window, the data can be averaged. If however a data point has always been measured outside the window, the data point which deviates least from the marked position can be used for the reconstruction.
In prospective respiratory gating the physiological respiratory signal measured using a respiratory sensor (e.g. the diaphragm position measured using a navigator sequence) is analyzed during the measurement and the MR measurement is controlled on the basis of the physiological signal captured. In the simplest embodiment, known as the Acceptance/Rejection Algorithm (ARA), the measurement of an imaging data packet (and if appropriate the assigned navigator sequence) is repeated until the physiological signal falls in a predefined acceptance window.